1Using gcov with the Linux kernel
2================================
3
4gcov profiling kernel support enables the use of GCC's coverage testing
5tool gcov_ with the Linux kernel. Coverage data of a running kernel
6is exported in gcov-compatible format via the "gcov" debugfs directory.
7To get coverage data for a specific file, change to the kernel build
8directory and use gcov with the ``-o`` option as follows (requires root)::
9
10    # cd /tmp/linux-out
11    # gcov -o /sys/kernel/debug/gcov/tmp/linux-out/kernel spinlock.c
12
13This will create source code files annotated with execution counts
14in the current directory. In addition, graphical gcov front-ends such
15as lcov_ can be used to automate the process of collecting data
16for the entire kernel and provide coverage overviews in HTML format.
17
18Possible uses:
19
20* debugging (has this line been reached at all?)
21* test improvement (how do I change my test to cover these lines?)
22* minimizing kernel configurations (do I need this option if the
23  associated code is never run?)
24
25.. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
26.. _lcov: http://ltp.sourceforge.net/coverage/lcov.php
27
28
29Preparation
30-----------
31
32Configure the kernel with::
33
34        CONFIG_DEBUG_FS=y
35        CONFIG_GCOV_KERNEL=y
36
37and to get coverage data for the entire kernel::
38
39        CONFIG_GCOV_PROFILE_ALL=y
40
41Note that kernels compiled with profiling flags will be significantly
42larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported
43on all architectures.
44
45Profiling data will only become accessible once debugfs has been
46mounted::
47
48        mount -t debugfs none /sys/kernel/debug
49
50
51Customization
52-------------
53
54To enable profiling for specific files or directories, add a line
55similar to the following to the respective kernel Makefile:
56
57- For a single file (e.g. main.o)::
58
59	GCOV_PROFILE_main.o := y
60
61- For all files in one directory::
62
63	GCOV_PROFILE := y
64
65To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL
66is specified, use::
67
68	GCOV_PROFILE_main.o := n
69
70and::
71
72	GCOV_PROFILE := n
73
74Only files which are linked to the main kernel image or are compiled as
75kernel modules are supported by this mechanism.
76
77
78Files
79-----
80
81The gcov kernel support creates the following files in debugfs:
82
83``/sys/kernel/debug/gcov``
84	Parent directory for all gcov-related files.
85
86``/sys/kernel/debug/gcov/reset``
87	Global reset file: resets all coverage data to zero when
88        written to.
89
90``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcda``
91	The actual gcov data file as understood by the gcov
92        tool. Resets file coverage data to zero when written to.
93
94``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcno``
95	Symbolic link to a static data file required by the gcov
96        tool. This file is generated by gcc when compiling with
97        option ``-ftest-coverage``.
98
99
100Modules
101-------
102
103Kernel modules may contain cleanup code which is only run during
104module unload time. The gcov mechanism provides a means to collect
105coverage data for such code by keeping a copy of the data associated
106with the unloaded module. This data remains available through debugfs.
107Once the module is loaded again, the associated coverage counters are
108initialized with the data from its previous instantiation.
109
110This behavior can be deactivated by specifying the gcov_persist kernel
111parameter::
112
113        gcov_persist=0
114
115At run-time, a user can also choose to discard data for an unloaded
116module by writing to its data file or the global reset file.
117
118
119Separated build and test machines
120---------------------------------
121
122The gcov kernel profiling infrastructure is designed to work out-of-the
123box for setups where kernels are built and run on the same machine. In
124cases where the kernel runs on a separate machine, special preparations
125must be made, depending on where the gcov tool is used:
126
127.. _gcov-test:
128
129a) gcov is run on the TEST machine
130
131    The gcov tool version on the test machine must be compatible with the
132    gcc version used for kernel build. Also the following files need to be
133    copied from build to test machine:
134
135    from the source tree:
136      - all C source files + headers
137
138    from the build tree:
139      - all C source files + headers
140      - all .gcda and .gcno files
141      - all links to directories
142
143    It is important to note that these files need to be placed into the
144    exact same file system location on the test machine as on the build
145    machine. If any of the path components is symbolic link, the actual
146    directory needs to be used instead (due to make's CURDIR handling).
147
148.. _gcov-build:
149
150b) gcov is run on the BUILD machine
151
152    The following files need to be copied after each test case from test
153    to build machine:
154
155    from the gcov directory in sysfs:
156      - all .gcda files
157      - all links to .gcno files
158
159    These files can be copied to any location on the build machine. gcov
160    must then be called with the -o option pointing to that directory.
161
162    Example directory setup on the build machine::
163
164      /tmp/linux:    kernel source tree
165      /tmp/out:      kernel build directory as specified by make O=
166      /tmp/coverage: location of the files copied from the test machine
167
168      [user@build] cd /tmp/out
169      [user@build] gcov -o /tmp/coverage/tmp/out/init main.c
170
171
172Note on compilers
173-----------------
174
175GCC and LLVM gcov tools are not necessarily compatible. Use gcov_ to work with
176GCC-generated .gcno and .gcda files, and use llvm-cov_ for Clang.
177
178.. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
179.. _llvm-cov: https://llvm.org/docs/CommandGuide/llvm-cov.html
180
181Build differences between GCC and Clang gcov are handled by Kconfig. It
182automatically selects the appropriate gcov format depending on the detected
183toolchain.
184
185
186Troubleshooting
187---------------
188
189Problem
190    Compilation aborts during linker step.
191
192Cause
193    Profiling flags are specified for source files which are not
194    linked to the main kernel or which are linked by a custom
195    linker procedure.
196
197Solution
198    Exclude affected source files from profiling by specifying
199    ``GCOV_PROFILE := n`` or ``GCOV_PROFILE_basename.o := n`` in the
200    corresponding Makefile.
201
202Problem
203    Files copied from sysfs appear empty or incomplete.
204
205Cause
206    Due to the way seq_file works, some tools such as cp or tar
207    may not correctly copy files from sysfs.
208
209Solution
210    Use ``cat`` to read ``.gcda`` files and ``cp -d`` to copy links.
211    Alternatively use the mechanism shown in Appendix B.
212
213
214Appendix A: gather_on_build.sh
215------------------------------
216
217Sample script to gather coverage meta files on the build machine
218(see :ref:`Separated build and test machines a. <gcov-test>`):
219
220.. code-block:: sh
221
222    #!/bin/bash
223
224    KSRC=$1
225    KOBJ=$2
226    DEST=$3
227
228    if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
229      echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
230      exit 1
231    fi
232
233    KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
234    KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
235
236    find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
237                     -perm /u+r,g+r | tar cfz $DEST -P -T -
238
239    if [ $? -eq 0 ] ; then
240      echo "$DEST successfully created, copy to test system and unpack with:"
241      echo "  tar xfz $DEST -P"
242    else
243      echo "Could not create file $DEST"
244    fi
245
246
247Appendix B: gather_on_test.sh
248-----------------------------
249
250Sample script to gather coverage data files on the test machine
251(see :ref:`Separated build and test machines b. <gcov-build>`):
252
253.. code-block:: sh
254
255    #!/bin/bash -e
256
257    DEST=$1
258    GCDA=/sys/kernel/debug/gcov
259
260    if [ -z "$DEST" ] ; then
261      echo "Usage: $0 <output.tar.gz>" >&2
262      exit 1
263    fi
264
265    TEMPDIR=$(mktemp -d)
266    echo Collecting data..
267    find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
268    find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
269    find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
270    tar czf $DEST -C $TEMPDIR sys
271    rm -rf $TEMPDIR
272
273    echo "$DEST successfully created, copy to build system and unpack with:"
274    echo "  tar xfz $DEST"
275